This invention relates to a burner for use in high-temperature industrial applications, and also includes a method for efficient burning of fuels at high temperatures.
Many industrial processes require burners capable of heating a furnace to a high temperature, above about 2000.degree. F. For example, it is known to melt scrap metal in order to recover pure lead for use in making batteries. The furnace needs to be kept at about 2000.degree. F., while the metal itself may be heated to 1200.degree. F. or more. Maintaining the furnace at these temperatures requires a burner having a flame temperature which is even higher, of the order of 3400.degree. F.
The invention is not limited to use in any one particular application; it can be used in many other high-temperature environments, especially where it is desired to accelerate the rate of heating.
Due to the increased concern about air pollution, and the need to comply with laws and regulations restricting atmospheric emissions, it is important that the combustion in a high-temperature furnace be complete. The U.S. Environmental Protection Agency, and various states, have established "opacity" standards governing the amount of particulates and unburned substances that may be vented into the atmosphere. The less complete the combustion, the more opaque the vented gas will be. The opacity problem is especially acute when a burner is used to melt scrap metal. Scrap metal generally contains deposits of oil and other organic impurities, which should be burned off during the melting process, but which will not be completely combusted unless there is sufficient oxygen and unless the temperature is sufficiently high. If the latter conditions are not met, the result is a residue of carbon monoxide and various other hydrocarbons, which manifest themselves as opaque emissions. If the fuel is oil instead of natural gas, the opacity problem is even greater.
One approach towards solution of the problem of incomplete combustion, and the resulting air pollution, is to enrich the combustion air with oxygen. By increasing the proportion of oxygen to about 26%, one increases the flame temperature from about 3400.degree. F. to about 3800-4000.degree. F. At these higher flame temperatures, the combustion rate is increased, and the combustion tends to be more complete.
One can also increase the completeness of combustion by reducing the flow of combustion air through the burner, while maintaining or increasing its oxygen content, by using an external source of oxygen. Reducing the flow of combustion air, while supplying oxygen from an external source, will reduce the proportion of inert gas (i.e. nitrogen) in the combustion atmosphere, and therefore increases the relative activity of the oxygen in that atmosphere. The result is a higher flame temperature, more complete combustion, and increased burner efficiency. To prevent the flame temperature from increasing to a dangerous level, one can reduce the flow of fuel into the burner.
Although the above-described approaches do improve the efficiency of burning, both in reducing the time needed to perform the desired melting or other operation, and in reducing the amount of pollutants emitted, they do not solve the problem entirely. Indeed, merely increasing the oxygen level to 26% has been found not to be sufficient to comply with current opacity standards
Another approach has been to use pure or substantially pure oxygen as the combustion medium. With pure oxygen, the flame temperature may be as high as 5000.degree. F. The use of pure oxygen will achieve complete combustion, and will reduce the opacity of the combustion products sufficiently to satisfy current governmental requirements. But using pure oxygen is expensive. The stoichiometric relationships are such that a great quantity of pure oxygen would be needed to burn the desired quantity of fuel. Moreover, the capital costs associated with burning in pure oxygen are substantial. One needs a burner that can withstand the very high temperatures generated by burning in pure oxygen. Also, the furnace lining must be able to withstand the high temperatures. One also needs a special control system to avoid melting the tip of the burner. Such a system generally includes a water distribution system to cool the tip, and this system must be automatic and fully integrated with the other components.
One might also attempt to achieve complete combustion by injecting oxygen directly into the furnace. But such a procedure will lower the temperature of the furnace, and therefore is not a solution to the problem.
The present invention includes a high-temperature burner whose efficiency is substantially comparable to that of a pure oxygen burner, but which is relatively inexpensive to build and operate. Indeed, the burner of the present invention can be made by retrofitting existing equipment, so the initial cost of practicing the invention is usually very small. The use of the burner of the present invention makes it feasible to raise the flame temperature, and the oxygen content of the combustion atmosphere, to enable the contents of a furnace to be melted without polluting the air.